Evidence suggests that the phospho-protein, phosducin, is a regulator of many G protein systems, including visual signal transduction in vertebrate photoreceptors. The long-term objective of this research is to understand the precise mechanism of phosducin regulation of G proteins in photoreceptors and other cells. Phosducin acts on the beta/gamma complex of G proteins to inhibit its function in the activation of G protein alpha subunits and effector proteins. When phosducin is phosphorylated, it does not affect these functions. This proposed mechanism of phosducin action has been termed the G beta/gamma sequestration model. The specific aims of this proposal are designed to test this model by: (1) testing the predictions of the G beta/gamma sequestration model on the electrophysiological responses of intact rods to determine if the effects of phosducin on G beta/gamma observed in vitro are also functional in vivo; (2) determining structurally how phosducin is inactivated by phosphorylation and (3) characterizing the cellular factors that determine the phosphorylation state of phosducin. The methods to be employed to achieve these aims are multi-disciplinary. They include biochemical techniques for measuring phosphorylation-dependent changes in phosducin structure, site-directed mutagenesis of key phosducin residues, collaborative electrophysiological studies under conditions of high and low phosducin activity and biochemical measurements of the effect of dopamine on phosducin activity. Defects in G protein pathway components cause retinopathies and other non-retinal diseases. Therefore, it is imperative to understand how G protein signaling works in order to identify the causes of such diseases and develop treatments. These studies contribute to the understanding of G protein function by addressing the role of phosducin in G protein pathway regulation.